The process and the device may be applied to many fields, notably to the survey of oil-bearing reservoirs. The evaluation of the wealth and of the productivity of a subsurface reservoir is achieved for example by numerical simulations from the measured values of some parameters of the rocks such as the capillary pressure and the relative permeability thereof for the fluids present therein. These parameters are generally measured in a laboratory from rock samples taken in situ by coring.
One method for measuring the capillary pressure consists for example in subjecting porous rocks saturated with liquid to a progressive-speed centrifugation and in measuring the amount of fluid produced as a function of the speed.
A conventional method is known for taking measurements on a saturated sample successively in a draining phase and in an imbibition phase. The sample, previously saturated with a determined fluid, is placed in an enclosure, on a porous plate letting through only the fluid, and another fluid under pressure is injected so as to expel progressively the initial fluid. The fluid expelled from the sample is collected on the other side of the porous plate. During an imbibition phase, the centrifugal pressure is decreased so as to survey the return of the initial fluid in the sample.
French Patent application FR-2,666,147 (U.S. Pat. No. 5,253,529) mentions a known centrifuging system adapted for receiving a sample of a porous material containing a fluid (water for example). This centrifuging system includes a motor driving several arms into rotation. The samples, in the shape of cylindric bars possibly sheathed on the circumference thereof, are placed in buckets containing another fluid such as oil for example, respectively arranged at the end of the arms so that the centrifugal force draws the denser fluid away radially from the samples. The radial progression of the fluids inside the bar is determined by measuring the variations in the time of propagation of acoustic waves through the bar as the fluids move. Several couples of transducers transmitting and receiving ultrasounds are arranged opposite each other in various places along each bucket. The drained fluids flow into a free cavity provided in the peripheral part of the bucket.
With the device described in the above-cited patent, the phenomenon of expulsion of a fluid from a porous rock as the centrifugal force increases may be easily determined. But the inverse phenomenon of spontaneous imbibition of a core sample by the fluid previously drained, which may provide additional data on the material, cannot be observed because there is generally no more contact between the sample and the interface between the two fluids, and the denser fluid can therefore not re-imbibe it during the deceleration phase.